1. Field of the Invention
This invention relates to a process for using an eddy current probe to accurately determine the location of the top and bottom edges of plates used to uniformly space the heat exchange tubes in a nuclear steam generator.
2. Description of the Prior Art
Processes for using eddy current probes to locate the positions of baffle plates and support plates in nuclear steam generators are known in the prior art. An example of the use of such a probe is disclosed in U.S. patent application Ser. No. 567,107, filed Dec. 30, 1983, and assigned to Westinghouse Electric Corporation, the entire specification of which is hereby expressly incorporated herein by reference. Such eddy current probes are particularly useful in implementing maintenance procedures in nuclear stem generators which require the radial expansion of selected portions of heat exchanger tubes. To fully appreciate the role played by such probes in these procedures, some background in the maintenance problems and procedures associated with such steam generators is necessary.
Nuclear steam generators generally include a primary side through which hot, radioactive water from the reactor core is admitted into a plurality of heat-exchange tubes which may be either U-shaped or straight. Such generators further include a secondary side which houses and spaces these tubes and circulates a flow of non-radioactive water therethrough so that non-radioactive steam may be generated from the energy output of the reactor core. The tubes of the steam generator transfer heat from the radioactive water flowing through their inside surfaces to the non-radioactive water flowing over their outside surfaces. To facilitate this heat transfer, a plurality of horizontally disposed baffle plates and support plates are mounted throughout the secondary side of the steam generator in order to uniformly space the heat-exchanging tubes from one another. Each of these plates includes a plurality of uniformly-spaced openings (which may be bores) through which the tubes extend. The openings of the plates are arranged in registry with one another in order that the heat-exchange tubes may be arranged parallel to one another. In order to increase thermal contact between the outside surfaces of the heat-exchange tubes and the non-radioactive water flowing through the secondary side inlet, the baffle plates of the generator are placed in a staggered arrangement to cause the water circulating through the secondary side of the generator to weave back and forth in a serpentine pattern. Unfortunately, the fluid currents associated with such a tortorous flow of water sometimes causes these tubes to vibrate against the walls of the bores in both the baffle plates and the support plates. The resultant mechanical shock can dent the tubes, and cause wear scars by disrupting the crystalline structure of the Inconel which typically forms these tubes. These dents and wear scars render the outside surfaces of the tubes subject to corrosion degradation from the salts and sludges which are present in the non-radioactive water which flows through the secondary side of the generator. Ultimately, the vibration caused by the heat exchange tubes rattling against the sides of the bores in the baffle plates and support plates may cause a significant number of these tubes to corrode until they crack, and radioactively contaminate the water flowing through the secondary side of the steam generator.
In order to solve the problems associated with such destructive tube vibration, maintenance procedures have been developed wherein the mandrel of a hydraulic expansion unit is inserted up through the vibrating tube in order to radially expand the tube in the vicinity of the bores of the baffle plates and support plates. This radial expansion in the tube reduces or eliminates the gap between the outer walls of the tube and the walls of the bore or other opening in the plate through which it extends, thereby preventing the opportunity for the tube to rattle against the walls of the bores. However, the successful implementation of such tube expansions relies in part on the proper axial alignment of the hydraulic expansion mandrel with the region of the tube surrounded by the bore of the plate. If the center of the expansion mandrel is not closely aligned with the center line of the plate surrounding the tube, part of the radially expanded walls of the tubes may bulge over either the top or bottom of the plate, thereby creating a substantial amount of strain in the crystalline structure of the Inconel forming the heat exchange tubes. Experience has shown that the strained areas of such tubes are more susceptible to the corrosion degradation which can lead to cracking.
In order to properly align the hydraulic expansion mandrels within such tubes so that the tubes are radially expanded without such destructive bulging, the prior art has employed eddy current probes utilizing a pair of axially aligned sensing coils. When these coils are connected to commercially available eddy current circuitry, wherein the current differential between the two coils is accurately monitored by means of an oscilloscope connected across an impedance-type bridge, a null-point signal will be generated when the coils are symmetrically diposed about the center line of the plate. Unfortunately, the accuracy of this null-point signal is dependent upon the existence of complete geometric symmetry in the bore through the plate. Inaccuracies may arise if one of the edges of the bores is chamfered or bevelled while the other edge is not. Additionally, such a null-point signal is capable of positively indicating the position of only one of the plate edges (assuming that the coils are spaced apart the same distance as the thickness of the plate).
Hence, there is a need for a process wherein an eddy current probe can more accurately detect the position of both the edges of a plate in a steam generator in order that a hydraulic expansion mandrel may be properly positioned incident to a tube expansion. Moreover, in view of the potentially destructive effects associated with tube bulging, there is a need for a process wherein an eddy current probe accurately and conveniently detects when a tube expansion has resulted in a bulge condition so that potential corrosion problems associated with the bulge may be prevented either by sleeving or plugging operations.